The present invention relates to a direct voltage-to-direct voltage converter including a transformer whose primary windings are connected periodically, via first and second electronic switches and with alternating polarity, to a constant voltage source and whose secondary windings are connected to a load via rectifying elements, with the electronic switches receiving turn-on pulses at a preselectable repetition rate from a control device which is synchronized by a clock pulse generator.
Siemens-Zeitschrift [Siemens Magazine] 48 (1974) No. 11, pages 840 to 846, specifically FIGS. 2 and 3, discloses a voltage converter operating according to the resonance principle. The direct voltage converter disclosed therein includes a resonance capacitor which is connected, via the respectively conducting first or second electronic switch, in series with the input voltage source and the primary winding of the transformer. The repetition rate with which the two electronic switches are alternatingly switched on is selected to be exactly as high as the resonant frequency of the resonant circuit formed of the resonance capacitor and the series inductance of the transformer.
With a square wave voltage, this converter generates a sinusoidal current whose zero passages coincide with the points at which the voltage is switched. In this converter operating according to the current resonance principle, the efficiency is high and the peak current stress on the electronic switches is low. However, it requires additional components, for example a resonance capacitor whose resistance losses reduce efficiency. Moreover, the repetition rate for the turn-on pulses cannot be selected freely but is determined by the characteristics of the resonance capacitor as well as by the inductance of the transformer, which is undesirable for many applications. High demands with respect to alternating current stability and capacitance value stability are placed on the resonance capacitor. Moreover, in the described converter there exists the danger of overloading the electronic switch during polarity reversal, i.e. during the transition from opening the first electronic switch to closing the second electronic switch.